1. Field of the Invention
This invention is directed to continuous pickling operations, and in particular, it is directed to apparatus and a method for improving surface quality in continuous hot-rolled steel sheet product by removing surface stains that may appear on the finished steel product if the pickling line needs to be stopped for any one of a variety of reasons. For example, during normal pickling operations new coils of steel sheet product are repeatedly spliced onto the trailing end of sheet steel product running through the pickle line. If the length of the sheet steel product held within the looper section of the line is not long enough to provide the necessary time to complete the splice, the line must be stopped. Additionally, pickle lines are routinely stopped for unscheduled maintenance and/or repair. They are also stopped to make minor line adjustments, and at times are even stopped because of power failures. During such line stops, long sections of pickled and partially pickled steel sheet product, for example up to 100xe2x80x2 or more in length, are disproportionately exposed to the pickle liquor and/or acid fume above the pickling tanks. Such prolonged exposure results in the reformation of iron oxides, or stains, onto the surface of the sheet product.
2. Brief Description of the Related Art
Various attempts have been made in the past to remove stains from pickled sheet steel product. One such past attempt is disclosed in U.S. Pat. No. 5,916,379 granted to Varley, et al. The patent is directed to a composition and method of using the composition to prevent surface staining (flash rusting) on steel sheet in a continuous pickling line. The inventors disclose using an aqueous solution comprising about 3%-15% by weight each of gluconate and citrate to prevent the formation of flash rust during a line stop. The rust inhibiting solution is applied in the rinse zone of the pickle line. Such rinse zone stain removers are referred to in the art as xe2x80x9cpost-treatmentxe2x80x9d stain removers.
Another post-treatment stain remover process, disclosed in U.S. Pat. No. 5,837,061, prevents formation of hydrolysis stains on sheet steel product. The post-treatment process includes proving an inert gas or CO2 atmosphere in the rinse zone of the pickle line to prevent staining.
A third Patent, U.S. Pat. No. 5,296,042, recognizes various problems associated with staining in pickling operations. The Patent teaches that although many compositions are known to remove stain from sheet steel, it is extremely difficult to effectively apply stain-removing compositions to sheet steel surfaces in a continuous high speed manufacturing line. The inventors further state that when hot rolled sheet steel is pickled with HCl, the product can suffer from iron and chloride staining during line stops, and that the efficiency of stain removing chemicals is directly related to rinsing techniques. The Patent teaches that xe2x80x9crinsing is of major importancexe2x80x9d in any process that attempts to remove the residual chloride ions that contribute to the formation of stain on pickled stain surfaces.
Another U.S. Pat. No. 4,389,254, discloses that when inorganic acids are used as a pickle liquor, it tends to cause surface marks and pitting on the steel sheet product. In order to overcome this problem, the inventors use carefully selected organic pickle liquor in their pickle tanks, for example formic acid, acetic acid, or citric acid.
U.S. Pat. No. 4,361,444 discloses a post pickling rinse process that includes a rapid and uniform air drying step to prevent water spotting on strip.
Finally, U.S. Pat. No. 1,902,815 is directed to a process for pickling sheet steel in a sulfuric acid bath at a temperature of about 200xc2x0 F. The pickled product is then squeezed between wringer rolls, rinsed with water sprays, and the rinsed sheet is given a final wipe between a last set of wringer rolls. A blast of hot air from upper and lower air blast nozzles provides a final drying step. The Patent discloses providing a continuous forced blast of hot air against the sheets to dry all water from both surfaces.
It appears that the more effective stain removing processes developed in the past include a post-treatment stain remover used in combination with spray or immersion type rinse steps. We have found, in actual continuous hot rolling operations, that such past stain removing processes still fail to completely remove all surface staining caused by pickle line stops. For instance, we have found that even the best post-treatment stain removers tend to leave about a 1-inch wide brown stain along both edges of the pickled sheet after the drying step is completed. It is believed that such staining is caused by the retention of residual stain remover solution along the edges of the continuous pickled sheet steel product after the drying operation is completed. We have also discovered that when we use such post-treatment stain removers in combination with an improved post-rinse strip drying process developed at our operation, stain removal is improved to where the 1-inch brown stain is completely removed or at least reduced to where it has little effect on the product. For example, when we use a post-treatment stain remover, such as a new Crown formula sold as (NCI-SRU), in combination with the improved air knife arrangement and drying-process of the present invention, the 1-inch brown stain is completely removed. The new air knife arrangement is located downstream of the last wringer roll in of the line, and it delivers a fan shaped blast of drying air that radiates outward from the center of the steel sheet product toward both continuous edges of the sheet. The blast of drying air from the air knives impacts upon the last wringer roll to prevent water xe2x80x9cdrag-outxe2x80x9d along the downstream edge of the roll, and the outward radiating blast of drying air drives any excess rinse toward and off the continuous edges of the pickled sheet product.
It is therefore an object of the present invention to provide an improved process for removing stains from a pickled sheet product.
It is a further object of the present invention to provide an improved rinse/dry process that eliminates edge staining on a pickled sheet product.
It is another object of the present invention to provide an improved spray and air knife arrangement that removes residual stain remover solution from the edges of a pickled sheet product.
In satisfaction of the foregoing objects and advantages, the present invention provides a continuous pickle line that includes an improved air knife arrangement adapted to provide an airflow along the surface of a continuous steel sheet processed in the continuous pickle line. The airflow impacts against the exit side of the roll gap located between a set of wringer rolls that contact the continuous steel sheet, and the force of the airflow impacting against the roll gap is sufficient to prevent rinse water drag-out between the set of wringer rolls and onto the dried surface of the continuous sheet being processed. The airflow in combination with a post-treatment stain remover applied in the rinse zone of the continuous pickle line improves surface quality when the continuous sheet being processed is subjected to a line stop.